1. Introduction to G410 MOUNTAIN WEATHER FORECAST FOR THE OLYMPICS WASHINGTON CASCADES AND MT HOOD AREA NORTHWEST WEATHER AND AVALANCHE CENTER SEATTLE WASHINGTON 230 PM PST MON JAN 06 2009 WEATHER SYNOPSIS FOR TUESDAY AND WEDNESDAY Substantial moisture continues to stream over a very flat offshore ridge early Tuesday afternoon, with a very strong jetstream of 150-170 mph pushing the trailing portion of a strong warm front over the region. This should be akin to opening the main valve on a very large fire plug and should result in increasingly heavy rain or snow in most areas Tuesday afternoon at substantially rising freezing levels and increasingly strong winds. As the offshore ridge flattens further and moves over the NW coast late Tuesday, the front is expected to stall over the region Tuesday night and much of Wednesday as minor disturbances ripple along its back edge offshore…

2. Four factors that affect the formation and release of avalanches

3. Why do we have to learn this?  Provides a better understanding of physical processes governing snowpack development metamorphism, and melt  Makes sense of weather observations and forecasts

4. Warm Air Rises & Cold Air Sinks Simple Picture

5. Differential heating of the Earth surface produces motion in the Earth's atmosphere.

6. H & L pressure circulation drives the wind Global Scale: High and Low pressure systems develop

7. Atmospheric Pressure: Force exerted by the weight of the air Mean sea level pressure for June, July, August, top and December, January, February, bottom.

8. Location of high or low pressure centers provides information about: Wind direction Wind speed Cloud cover Precipitation Discrete Air Masses

17. Differences in pressure from one location to another causes: 1)Horizontal motions (wind) 2)Vertical motions (convection and subsidence)

18. The change in pressure measured across a given distance is called a "pressure gradient".

19. Northern Hemisphere Winds blow clockwise out of a high pressure center. Winds blow counterclockwise into a low pressure system 05 Jan 2009 500 mb

20. High Pressure Centers A high pressure center is where the pressure has been measured to be the highest relative to its surroundings. That means, moving in any direction away from the "High" will result in a decrease in pressure. A high pressure center is indicated on a weather map by a blue "H".

21. Low Pressure Centers A low pressure center is where the pressure has been measured to be the lowest relative to its surroundings. A low pressure center is indicated on a weather map by a red "L" and winds flow counter- clockwise around a low in the northern hemisphere.

23. As air flows out of a high pressure center, air from higher in the atmosphere sinks to replace it. This subsidence produces warming, dissipation of clouds and precipitation As air converges in a low pressure center, it rises and cools. If moist, cooling will cause moisture to condense and form clouds.

24. Rising pressure readings: approach of a high pressure center and fair weather. Falling pressure readings: approach of a low pressure center and stormy weather. Mountain barriers influence the development of high and low pressure centers Intense low or high pressure systems often occur on lee side of mtn. barriers

25. Pressure decreases with height Falling pressure readings: approach of a low pressure center and stormy weather. Pressure gradient maps = semi-horizontal layer As surface of constant pressure descends, the pressure measurement rises Rule of Thumb 10 m height change = 1 mb Top of Space Needle to Base: 184 m --- 22 mb Mt. Everest: 8848 m w/ 314 mb pressure at summit 31% of atmosphere is present above this level.

26. Pressure level maps (rather than heights)  Surface maps, at Earth surface  850 mb - available moisture for precipitation, ~1500m level  500mb - middle of atmosphere, troughs (lows) and ridges (highs), ~5500 m level  300 mb - jet streams at ~9100 m level [The average atmospheric pressure at sea level is ~1000mb]

27. A line connecting points of equal pressure is called an isobar. At every point along a given isobar, the values of pressure are the same.

28. Pressure reports in Washington are between 1004 mb and 1020 mb. An area of relatively high pressure is centered in western S Pacific, while the pressure increases outward from this region. An area of lower pressure is located in western B.C. & Ak.

29. Troughs: Elongated low pressure centers Ridges: Elongated high pressure centers. The small green numbers are contour labels, which identify the value of an isobar.

31. Small wave length waves or ripples are called short- wave troughs or ridges. 500 mb forecast for 1/21/1999. Troughs = red; solid lines = 500 mb height contours.

32. High and low pressure centers and Wind direction Primary force = pressure gradient force (differences in pressure are proportional to pressure gradient Pressure gradient force is directed from high pressure to low pressure, crossing isobars at right angles. Secondary force = Coriolis force, apparent force from earth’s rotation Deflects winds at 90° to the right in direction of travel (N. hemisphere) Above 1000m, the balance of pessure forces cause winds to blow parallel to isobars or pressure height contours

33. High and low pressure centers and Wind direction Path of air parcel starting out from rest in upper levels. Air flows towards low pressure Air turns to the right by CF When CF and PGF balance, winds blow parallel to the isobars. These winds are called geostrophic winds. Geostrophic wind blows parallel to the isobars because the Coriolis force and pressure gradient force are in balance. A. Aloft & B. at the ground Different wind directions and speeds relative to height contours (aloft) and pressure contours (ground).

34. High and low pressure centers and Wind direction Winds at ground blow more directly into low pressure centers. Monitor changes in wind direction, can see changes of pressure patterns and movement of high and low pressure centers

35. Surface Maps Uses available surface data Shows:  Temperature  Weather  Dew point temperature  Cloud cover  Pressure  Wind direction and speed (wind barb)

38. Observed Dew Point Temperature Observed Temperature station reporting symbol

39. Surface Maps Cloud Cover: The amount of cloud cover at the time the observation. In this case, broken clouds were reported. Sea Level Pressure: The last three digits of the sea level pressure reading in millibars (mb). Wind Barb: The wind barb. The wind barb indicates wind direction and wind speed.

40. Surface Maps Use available surface data Shows: Temperature Weather Dew point temperature Cloud cover Pressure Wind direction and speed (wind barb)

41. Dew Point and RH When the dew point temperature and air temperature are equal, the air is saturated. Dew point temperature is NEVER GREATER than the air temperature. If the air cools, moisture must be removed from the air. this is accomplished through condensation. Condensation is the formation of tiny water droplets that can lead to the development of fog, frost, clouds, or even precipitation.

42. Dew Point and RH The higher the moisture content of the air at a given temperature. Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach saturation. A state of saturation exists when the air is holding the maximum amount of water vapor possible at the existing temperature and pressure.